A generic circuit arrangement is known from U.S. Pat. No. 5,777,468 A. When such a circuit arrangement is used to determine the weight of a person sitting in a driver's seat—i.e. the force being exerted upon the driver's seat—the amount of displacement of the driver's seat caused by the exertion of force or by the weight of the person is measured and converted into an electrical signal. In such an application it is desirable not only for the person's weight to have a sufficiently high resolution, but also for the seat to be capable of moving by only a few millimeters, to prevent any uncomfortable rocking motion being caused by the required sprung suspension. This means it is usually necessary to use a very firm spring, for example in the form of a flat spring.
A further circuit arrangement with a linear variable differential transformer is known from U.S. Pat. No. 5,180,979, in which the primary coil is triggered with a triangular voltage. The resulting total triangular voltage on the two secondary coils is compared to a reference voltage and the amount by which the total voltage duration exceeds the reference voltage duration is indicative of the magnitude of the displacement of the transformer core. The required minor displacement combined with a high resolution therefore leads to a requirement for high sensitivity in the measurement device.
The measurement signal of the circuit arrangement, which is proportional to the force exerted, is dependent upon the temperature to which the circuit arrangement is exposed. It is therefore necessary to determine the dependency of the measurement signal on the temperature and to perform a corresponding correction for any prevailing temperature. This means, however, that it is necessary to know the current temperature, and therefore a thermal detector may be used to measure the temperature. The use of additional thermal detectors, however, requires additional expenditure on materials and is therefore less cost-effective.
A method for determining the temperature of an inductive displacement sensor by calculating the temperature-dependent copper resistance of the coil winding is known from DE 43 13 273 A1. In the known method, a constant voltage is applied and maintained on the serial connection consisting of the coil inductance and a resistor until the inductance reaches saturation, so that the current through the coil is still only determined by the copper resistance. The current coil resistance and thus the current temperature are determined from the resulting voltage on the coil. However, in a circuit arrangement with a transformer, whose primary coil has a triangular voltage applied to it and in which a triangular voltage should also be present on the secondary side, this measurement principle cannot be used since the coil cannot reach saturation.